1. Field of the Invention
The present invention relates to field emitters and methods of fabricating the same. More particularly, the present invention relates to forming field emission tips by the use of facet etching.
2. State of the Art
Various types of field emitters are used in a variety of devices, from electron microscopes to ion guns. However, one of the most prevalent commercial applications of field emitters is flat panel displays, such as cold cathode field emission displays (xe2x80x9cFEDsxe2x80x9d) used for portable computers and other lightweight, portable information display devices.
As illustrated in FIG. 18, an exemplary flat panel cold cathode FED 200 comprises a flat vacuum cell 202 having a cathode 204 and an anode 206 spaced apart from one another in a mutually parallel relationship. The cathode 204 comprises a conductive or semiconductive first material 208, such as silicon, disposed on a substrate 212, such as a semiconductive or dielectric material, and an array of minute field emission tips 214 distributed across the material 208. The anode 206 comprises a conductive second material 216 disposed on an interior surface of a transparent plate 218 and a phosphorescent or fluorescent material 222 coated on the conductive second material 216. A conductive structural element, called a gate 224, is disposed in the space between the cathode 204 and anode 206. The gate 224 is generally formed atop a grid of dielectric material 226 deposited on the cathode 204. The field emission tips 214 reside within openings in the gate 224 and in the dielectric material 226, such that the gate 224 surrounds each field emission tip 214. The gate 224 acts as a low-potential anode (i.e., lower potential than the anode 206), such that when a voltage differential, generated by a voltage source 228, is applied between the cathode 204 (strong negative charge), the gate 224 (weak positive charge), and the anode 206 (strong positive charge), a Fowler-Nordheim electron emission is initiated, resulting in a stream of electrons 232 being emitted from the field emission tips 214 toward the phosphorescent or fluorescent material 222. The electron stream 232 strikes and stimulates the phosphorescent or fluorescent material 222. The stimulated phosphorescent or fluorescent material 222 emits photons (light) (not shown) through the conductive second material 216 and the transparent plate 218 to form a visual image.
FIGS. 19-23 illustrate a conventional method of forming a field emission tip. As shown in FIG. 19, a substrate of conductive or semiconductive material 252, such as silicon, is deposited or formed over a dielectric support 254. A mask material is patterned (such as by lithography) to define a mask element 256 at the position of the emission tip 258 to be formed. The conductive or semiconductive material 252 is then etched, such as by a wet etch or an isotropic dry etch, which xe2x80x9cundercutsxe2x80x9d the mask element 256 to form a sharp field emission tip 258 beneath the mask element 256, as shown in FIG. 20. The mask element 256 is then removed, as shown in FIG. 21. Although such a method is commonly used to form field emission tips 258, the method has drawbacks. For example, as shown FIG. 22, if the etching is halted too soon, inefficient, blunt field emission tips 262 are formed. Further, if the etching is not halted soon enough, the mask element 256 is undermined and the field emission tips 264 formed are short and may be ineffective, as shown in FIG. 23 (shown with the mask element 256 collapsed onto the conductive or semiconductive material 252). In other words, the short field emission tip 264 may not be close enough to a gate in a field emission display to generate a sufficient stream of electrons striking the phosphorescent or fluorescent material on the anode to stimulate the material and form a visual image.
Other field emission tip formation techniques which do not involve isotropic etching are also known. For example, U.S. Pat. No. 5,312,514, issued May 17, 1994 to Kumar (xe2x80x9cthe Kumar patentxe2x80x9d), relates to forming field emission tips by distributing a discontinuous etch mask material across an electrically conductive material layer. The discontinuity of the etch mask material forms random openings therein. The etch mask material is selected such that the electrically conductive material layer will etch at a faster rate than the etch mask material (at least twice the rate) when the electrically conductive material layer is ion etched. The ion etch is performed until all of the etch mask is removed, which results in v-shaped valleys in the electrically conductive material defining peaked field emission tips therebetween. Further, the Kumar patent discusses using a low work function material for the electrically conductive material layer and also discusses depositing a low work function material over the electrically conductive material after the formation of the field emission tips. Although the method taught in the Kumar patent eliminates the use of an isotropic etch to form field emission tips, it lacks control over the field emission tip distribution and dimensions. The discontinuous layer of etch mask material results in a nonuniform distribution of field emission tips, since the positions of the openings in the discontinuous layer cannot be controlled. Furthermore, the discontinuous layer of etch mask material results in non-uniform dimensions between the field emission tips, since the thickness difference across the discontinuous layer cannot be controlled. In other words, the field emission tips formed in areas where less etch mask material existed over the conductive material will be shorter than in other areas. Moreover, since the etch mask material is a discontinuous layer rather than a patterned mask, the size or diameter of the field emission tips formed cannot be controlled.
Thus, it can be appreciated that it would be advantageous to develop a technique which would result in novel field emission tips having uniform distribution and uniform, precise dimensions.
The present invention relates to field emitters and methods of fabricating the same, wherein the field emission tips of the field emitters are formed by utilization of a facet etch.
In an exemplary method of the present invention, an etch mask is patterned on a conductive substrate material in the locations desired for subsequently formed field emission tips. The etch mask can be patterned in various shapes in order to achieve a desired field emission tip structure. For example, a circular mask element will result in a conical field emission tip, a triangular mask element will result in a tetrahedral field emission tip, a square mask element will result in a pyramidal field emission tip, and so on. The conductive substrate material is anisotropically etched to translate the shape of the mask into the underlying conductive substrate material, which forms a vertical column having a cross-section with the same shape as the mask element, from the conductive substrate material. The anisotropic etch is conducted for a predetermined duration of time, which will result in a column of a specific height required for the subsequently formed field emission tip. The etch mask element is then removed (optional) and the vertical column is facet etched to form the field emission tip.
The facet etching is generally performed in a chamber in which ions can be accelerated to strike a substrate, such as reactive ion etchers, magnetically enhanced reactive ion etchers, low pressure sputter etchers, and high density source etchers. As opposed to anisotropic etches, such as ion etching or plasma etching processes, in which ions strike the surface of the substrate substantially perpendicular to result in a vertical etch, a facet etch results in ions dispersed in a fashion which results in the ions striking 90 degree features (i.e., corners) of structures on the substrate at a rate which is about four to five times that of the rate at which ions strike substantially planar surfaces (e.g., surfaces laying substantially perpendicular to the ion emission source) on the substrate. In fact, with facet etching, the planar surfaces experience very little substrate loss. The facet etch creates a gradual slope of about 45 degrees at the corners of the structures on the substrate.
The facet etch is preferably performed in a reactive ion etcher wherein the substrate is placed on a cathode within a high-vacuum chamber into which etchant gases are introduced in a controlled manner. A radio frequency power source creates a plasma condition in the high-vacuum chamber which generates ions. The walls of the high-vacuum chamber are grounded to allow for a return radio frequency path. Due to the physics of the radio frequency powered electrodes, a direct current self-bias voltage condition is created at the substrate location on the cathode, which causes the generated ions in the plasma to accelerate toward and strike the substrate. The etchant gases utilized in the facet etch are preferably inert gases, including, but not limited to, helium, argon, krypton, and xenon. These inert gases have been found to enhance the uniformity of the facet etch process. It is, of course, understood that any other suitable gas or mixture of gases which are inert with respect to the material of the substrate may also be used.
Thus, the present invention eliminates the use of isotropic etching to form field emission tips and, thereby, eliminates the problems associated with isotropic etching. Although the present invention requires more steps than the typical isotropic etching technique of forming field emission tips, the methods of the present invention result in more uniform distribution, size, and height for the field emission tips, since the location and size of the etch mask elements defining the tip locations, as well as the depth of the anisotropic etch, can be precisely controlled. This precise control results in a field emission tip array having regular uniform tip spacing as well as precise, uniform tip height, thus improving the performance and reliability of the field emission display device formed therefrom. Furthermore, the precise control of the tip spacing allows the tips to be packed closer to one another, which results in a higher fidelity screen with more pixels per square inch.
The present invention also allows for low work function materials to be easily incorporated into the field emission tips. The overall work function of a field emission tip affects its ability to effectively emit electrons. The term xe2x80x9cwork functionxe2x80x9d relates to the voltage (or energy) required to extract or emit electrons from a field emission tip. The lower the work function, the lower the voltage required to produce a particular amount of electron emission. Thus, the incorporation of low work function materials in field emission tips can substantially improve their performance for a given voltage draw.
A variety of low work function materials can be incorporated into the field emission tips of the present invention. Such low work function materials include, but are not limited to, AlTiSix (aluminum titanium silicide [wherein x is generally between 1 and 4]), TiSixN (titanium silicide nitride), TiN (titanium nitride), Cr3Si (tri-chromium mono-silicon), TaN (tantalum-nitride), or the like. Moreover, other low work function materials, such as metals including cesium (Ce), and cermets including Cr3Sixe2x80x94SiO2 (tri-chromium mono-silicon silicon-dioxide), Cr3Sixe2x80x94MgO (tri-chromium mono-silicon magnesium-oxide), Auxe2x80x94SiO2 (gold silicon-dioxide), and Auxe2x80x94MgO (gold magnesium oxide), may also be used.
One embodiment of the invention for incorporating low work function materials into the field emission tips according to the present invention involves depositing a low work function material on a conductive substrate material. The low work function material may be deposited by ion beam sputtering, laser deposition, evaporation, chemical vapor deposition (CVD), and sputtering. An etch mask is then patterned on the low work function material to form discrete mask elements in the locations desired for the field emission tips to be formed. The low work function material and conductive substrate material are then anisotropically etched to form a column under each etch mask element from the conductive substrate material and a portion of the low work function material. The etch mask elements are then removed (optional). The vertical columns, capped with the low work function material, are then facet etched to form an array of low work function material-tipped field emission tips. Redeposition material, comprising a mixture of material from the vertical column substrate material and the low work function material, generated by the facet etch collects in corners at junctions of the vertical columns and the base conductive substrate during the facet etch.
Another embodiment of the invention for incorporating low work function materials into the field emission tips according to the present invention involves incorporating a sacrificial layer to assist the removal of redeposition material from the field emission tip. As with the previously discussed embodiments of the present invention, a low work function material is deposited on a conductive substrate material. An etch mask is patterned to form etch mask elements on the low work function material in the locations desired for the field emission tips to be formed. The low work function material and conductive substrate material are then anisotropically etched under such mask elements to form vertical columns from the conductive substrate material capped by a portion of the low work function material. The etch mask elements are then removed (optional). A sacrificial material, such as silicon dioxide or tetraethyl orthosilicate (TEOS), is then conformally deposited over the array of vertical columns, each capped with the low work function material, to form a covered structure. The covered structures are then facet etched to form an array of low work function material-tipped field emission tips. Redeposition material generated by the facet etch, comprising a mixture of material from the vertical column, the low work function material, and the sacrificial material, collects in exposed corners of the sacrificial material at a junction of the vertical column and the conductive substrate during the facet etch. Although such redeposition material would be difficult to remove if deposited directly on the conductive material of the tips and underlying substrate, the presence of the sacrificial material under the redeposition material allows the redeposition material to be easily removed using a clean-up technique, such as a hydrofluoric acid (HF) dip or a diluted HF dip, as known in the art. The mask element is then removed, as known in the art.
Thus, the present invention allows for easy incorporation of a variety of materials on top of the field emission tips to improve their performance.